From 87bc2cab4e6007c4d129ed0b97b54b7f47bd987b Mon Sep 17 00:00:00 2001
From: "Matt A. Tobin" <email@mattatobin.com>
Date: Thu, 16 Apr 2020 17:54:38 -0400
Subject: [PATCH] Bug 1343879 - Be consistent about the parent style context
 the document-level anonymous content container should get: it should get no
 parent style context.

Tag #1375
---
 layout/base/nsCSSFrameConstructor.cpp | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index e88b2d62c5..3747642034 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -10751,16 +10751,22 @@ nsCSSFrameConstructor::AddFCItemsForAnonymousContent(
       }
     }
 
-    if (inheritFrame->GetType() == nsGkAtoms::canvasFrame) {
-      // CorrectStyleParentFrame returns nullptr if the prospective parent is
-      // the canvas frame, so avoid calling it in that situation.
-    } else {
-      inheritFrame = nsFrame::CorrectStyleParentFrame(inheritFrame, pseudo);
-    }
-    Element* originating = pseudo ? inheritFrame->GetContent()->AsElement() : nullptr;
+    nsIFrame* styleParentFrame =
+      nsFrame::CorrectStyleParentFrame(inheritFrame, pseudo);
+    // The only way we can not have a style parent now is if inheritFrame is the
+    // canvas frame and we're the NAC parent for all the things added via
+    // nsIDocument::InsertAnonymousContent.
+    MOZ_ASSERT_IF(!styleParentFrame,
+                  inheritFrame->GetType() == nsGkAtoms::canvasFrame);
+    // And that anonymous div has no pseudo.
+    MOZ_ASSERT_IF(!styleParentFrame, !pseudo);
 
+    Element* originating =
+      pseudo ? styleParentFrame->GetContent()->AsElement() : nullptr;
+    nsStyleContext* parentStyle =
+      styleParentFrame ? styleParentFrame->StyleContext() : nullptr;
     styleContext =
-      ResolveStyleContext(inheritFrame->StyleContext(), content, &aState, originating);
+      ResolveStyleContext(parentStyle, content, &aState, originating);
 
     nsTArray<nsIAnonymousContentCreator::ContentInfo>* anonChildren = nullptr;
     if (!aAnonymousItems[i].mChildren.IsEmpty()) {